One embodiment of the invention relates to an acquisition apparatus and method for use in a mobile communication system using a direct-sequence code division multiple access (DS-CDMA) scheme.
In CDMA mobile communications, acquisition is the most basic function first required prior to a start of communications. Because acquisition requires a time constituting an overhead of communications, the speed increase thereof is important.
Acquisition is a method of establishing a synchronization by generating spreading codes (reference signals) in predetermined timing at a mobile station and matching those to the timing of spreading codes of from the base station while deviating the timing thereof. Specifically, by performing correlating operation while sequentially changing the phase of the spreading codes by use of the same spreading codes as used at the transmission side, the correlation value obtained by correlating operation is compared with a threshold so that synchronization can be established with the transmission side by utilization of the comparison result (see Non-patent Document A. Polydoros and C. L. Weber, “A unified approach to serial search spread-spectrum code acquisition—Part 1: General Theory,” IEEE Trans. Com., for example). The threshold, for comparison with the correlation value, is usually established by adjusting the accurate channel estimation value calculated from a reception signal.
Meanwhile, there is proposed an approach of controlling the threshold by calculating the mean power over reception signals and performing arithmetic operation on the calculated mean powers as an art to adapt the threshold, for comparison with a correlation value, to communication environment (see Patent Document JP-A-2001-2512161).
However, in acquisition, correlating operation and comparison with a threshold are repeated a plurality of times. Each time complicated operation is executed, operation amount increases significantly thus increasing the data processing amount and hardware scale.